Fix unreachable code mypy warnings in singularity provider

[yunchipang]

related: #53395

original warning:

providers/singularity/src/airflow/providers/singularity/operators/singularity.py:176: error:
Statement is unreachable  [unreachable]
[self.log.info](http://self.log.info/)("Stopping Singularity instance")

this is caused by:

def on_kill(self) -> None:
    if self.instance is not None:
        self.log.info("Stopping Singularity instance")
        self.instance.stop()

when mypy infers the type of self.instance as None, it makes the if self.instance is not None condition always false, which makes self.log.info("Stopping Singularity instance") unreachable.

fix: use self.instance: Any = None to tell mypy that self.instance can be None or any other type
note: affected # type: ignore[attr-defined] comments are removed to fix all warnings

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[jscheffl]

Fix sounds reasonable and is well analyzed. Please have another eye on review before merge.